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Dimly lit f-150

Hello, My wife has a 1993 f-150 xlt long bed that has a lighting problem. With the headlights on the low beams are really really dim, almost out. When i put the high beams on only one bulb goes on. I have replaced both bulbs and checked the plugs. One bulb plug has a slightly loose terminal but makes contact.

I was wondering if anyone has had or heard of this.

It does have dealer installed roof running lights and several years ago, like 8 or so, it had a short in the roof area which was repaired. This headlight problem has been with the truck for about 1 year or so. We drive during the day time 95% of the time but would like to fix it.

Any help would be great.
Thanks in advance.

Fix the bad contact first. A bad contact has higher resistance so will let less current through. It can also heat up and eventually melt the connector.
They all likely have a ground wire in common so trace figure out which one is ground (the one that doesn’t have 12V on it when on) and to where it actually meets a larger beefy part of the frame or some sort of ground junction. That connection could very well be bad.

If you don’t already own a voltmeter I suggest you purchase one so you can check how much voltage is getting to the lights. The trouble could be due to resistace on the power side of the circuit which will cause the lights to have less than near full battery voltage getting to them when they are on. If you have the correct voltage getting to the lights then check the voltage on the ground side of the light connection. If the ground is bad you will see more than .1 volt across the lead. Be sure to use a good ground connection for your meter reference. The trouble might be in the battery to chassis ground connection.